Athletic performance apparel has become increasingly popular as they provide specific benefits for a given physical activity. Some examples of such apparel include compression clothing, sports bras, yoga pants, running shoes, and cross-training shoes. For compression clothing, the principal functionality is to maintain a skin-tight fit, and can further apply pressure to parts of the body to prevent injury and improve blood flow during exercise, providing the benefit of an improved performance and recovery. For running shoes, the principle functionality is to provide sufficient stability and cushioning to absorb shock and to reduce the magnitude of localized pressure peaks, thereby providing the benefit of reduced stress to the foot and improved performance.
Athletic apparel is expected to develop further in the future, adding more functionality and benefits. This includes compression athletic wear that is customized to the athlete's dimensions and type of activity. With the increase in hyper-engineered apparel costs, consumers will demand perfection and want to know definitively when their apparel is deteriorating in performance. Fabric performance is also important in other contexts, such as bras, shapewear, and mattresses.
High performance fabric is made out of synthetic polymeric fibers. The unique material properties of polymeric fibers contribute to the desired performance of the fabric for athletic activity and/or support. Polymers are compliant, i.e., have a low elastic modulus, which contributes to the fabric's flexibility. Upon being stretched, polymers can store energy, which allows the polymeric fibers to return to their original state with force after an athletic movement.
However, the polymeric fibers lose their elastic recovery over time by undergoing irreversible deformation. The fibers stretch over time, but without storing energy that would allow the fibers to return to their original state. Therefore, the clothing loses its shape and the compressive performance of the fibers is compromised. Fatigue deformation of polymers is controlled by a viscoelastic phenomenon called creep and stress relaxation. On a micro scale, with continuously applied moderate force, the polymer chains slowly swim around each other and align with the direction of force. On a macro scale, the fibers stretch and thin out. Eventually, as the cross-sectional area of the fibers decreases, the polymer chains can no longer accommodate the force and bonds within the chains, and cross-links between the chains will break. Thus, the user may continue to use such apparel without knowing that its benefit has run its course due to prolonged exposure to a variety of factors, including, but not limited to, stretching, shear force, high temperature, and water.
For high performance footwear, the cushioning material embedded in the midsole, outsole, and heel help redistribute the ground resultant force acting on the body during physical activities such as walking, running, or jumping. The cushioning material used in the midsole can vary over different types, such as foam materials or polyurethane, while the outsole and heel are typically made from rubber. Similar to the polymers in high performance fabrics, the cushioning material in high performance footwear, when receiving a pressure load, can store energy when being elastically deformed, and thus allowing for an elastic recovery to its original shape when the pressure load is removed.
However, also similar to the fibers found in high performance fabrics, the cushioning in the shoe does not store all the energy from the ground resultant pressure force in its deformed state, but instead some energy is released as heat. Thus over time, the recurrence effects of activity and heat build-up within the cushioning causes the material to eventually denature, and thereby leading to a reduction in the elasticity of the cushioning material, ie stiffening. Moreover, the impact forces and friction forces, along with heat and water exposure, may result in a reduction in the thickness on the outsole and heel, thereby also reducing the shock absorbency and cushioning for the wearer. Thus, similar to high performance fabric, a user may continue to use a high performance footwear without realizing that benefits, such as absorbing shock and distributing stress points, are no longer available.
It should, therefore, be appreciated that there is a need for fabric and cushioning material that can each accurately alert the user when it is ceasing to perform adequately, particularly when it is a component of athletic apparel, wherein such alerting mechanism will be readily indicative and not be overly intrusive nor aesthetically displeasing to the apparel.